An engine fuel supply apparatus mixes fuel with air in a carburetor and supplies the resulting air-fuel mixture from the carburetor into a cylinder.
It is believed that when an engine is rapidly accelerated from idling (when the engine speed is rapidly increased), the supply of fuel is delayed due to a sudden change in the amount of air flow, the air-fuel mixture is temporarily diluted, and the engine is inadequately accelerated or stopped.
An engine fuel supply apparatus having an insulator provided between the engine and the carburetor to prevent engine heat from being transmitted to the carburetor, and also having a fuel booster pump provided to the insulator is known as a measure against the above-described problem, as is disclosed in JP 2007-071054 A (Patent Document 1). Providing a fuel booster pump allows the amount of fuel in the air-fuel mixture to be temporarily increased during engine acceleration.
The engine fuel supply apparatus of Patent Document 1 has an air-fuel mixture supply channel provided in the lower half of the insulator, an air channel provided in the upper half, and a fuel booster pump provided in the bottom part of the insulator. The air channel is communicated with a negative-pressure chamber of the fuel booster pump via an air introduction channel.
In this engine fuel supply apparatus, the air introduction channel is kept under negative pressure during idling because the throttle angle is small. The air introduction channel kept under negative pressure creates negative pressure in the negative-pressure chamber of the fuel booster pump. The negative-pressure diaphragm of the fuel booster pump is thereby moved toward the negative-pressure chamber by the elastic force of a spring member.
When the angle of a throttle valve is increased from this state and the vehicle is rapidly accelerated, air is fed to the air introduction channel and the negative-pressure chamber of the fuel booster pump. The negative-pressure diaphragm of the fuel booster pump acts against the elastic force of the spring member and moves instantaneously toward the pump chamber. Air in the pump chamber is forced out to a pressure chamber via a communicating channel.
The pressure diaphragm is pushed out toward the fuel chamber, and the fuel in the fuel chamber is supplied to the air-fuel mixture supply channel in a temporarily increased amount. The amount of fuel in the air-fuel mixture is thereby temporarily increased with a fast response in correspondence with an operation of the throttle valve when the engine is rapidly accelerated from idling.
In the fuel supply apparatus of Patent Document 1, however, an air channel must be provided to the upper half of the insulator in order to provide a fuel booster pump to the insulator. In other words, two channels (the air-fuel mixture supply channel and the air channel) must be provided to the insulator in the fuel supply apparatus, making it difficult to keep the device compact.
Further, because an air channel is provided to the upper half of the insulator, and a fuel booster pump is provided to the lower half of the insulator, an air-fuel mixture supply channel is provided between the air channel and the fuel booster pump. The air introduction channel for providing communication between the air channel and the fuel booster pump must therefore bypass the air-fuel mixture supply channel, the air introduction channel has a complex shape, and the overall length dimension is increased. The complex shape and increased overall length dimension of the air introduction channel sometimes delays the timing with which the air is fed to the negative-pressure chamber via the air introduction channel. For this reason, boosting and drawing the fuel from the fuel chamber with a fast response in correspondence with an operation of the throttle valve is difficult.